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Professor Atsushi Kume and his research group have discovered that gravitropic response, photosynthesis, and plant growth are regulated by the same gene, ISSUNBOSHI1
When plants first emerged from the water, they faced two problems: an increase in body weight due to the loss of buoyancy and a decrease in the efficiency of carbon dioxide uptake through their bodies. To support themselves on land, early land plants developed robust cell walls. However, strengthening the cell walls can inhibit the uptake of carbon dioxide from the atmosphere into the chloroplasts, which can hinder photosynthesis and growth. This would also reduce growth. Therefore, new genes may have emerged to address these issues. Using a model moss plant that retains a strong evolutionary memory, researchers evaluated the effects of stepwise increases in gravity up to ten times that of Earth on moss growth and photosynthesis. The moss plants were grown in environments with 3, 6, and 10 times Earth's gravity for eight weeks in a centrifuge and irradiated with light. The researchers found that the moss plants' photosynthetic rate and growth increased significantly.
At 6 and 10 times gravity, the size of the chloroplasts increased, the number of plants (phyllid gametophores) increased, and the chloroplasts became more accessible to carbon dioxide in the atmosphere, thereby improving the photosynthetic rate. Conversely, the length of the stems and leaves decreased. A transcriptome analysis (RNA-seq), which is a comprehensive genetic analysis of the entire genome, was conducted to investigate which genes were activated in response to changes in gravity. Notably, the expression of genes encoding AP2/ERF transcription factors, which are unique to mosses, increased markedly. When the expression of one of these transcription factors was artificially increased to enhance its function, the plant showed characteristics similar to those obtained under 10-fold gravity, such as shortening of stem and leaf length and promotion of photosynthesis under normal gravity. Conversely, when its expression was reduced, the opposite was observed. This transcription factor was named ISSUNBOSHI1 after a small hero in an old Japanese tale, because it has the "strong power" to increase photosynthesis in small mosses. AP2/ERF transcription factors, including ISSUNBOSHI1, may have played an important role in the evolution of land plants 500 million years ago.
AP2/ERF transcription factors are widely found in other land plants, but their functions remain unknown. Similar AP2/ERF transcription factors to those found in ISSUNBOSHI1 may exist in plants other than mosses. In the future, identifying such factors and enhancing their functions could lead to the development of useful plants with high photosynthetic capacity and yield. In addition, as human activities in space increase, understanding the growth mechanisms of plants in various gravitational environments is becoming an important new research area. These results are expected to deepen our understanding of basic life phenomena related to how plants adapt to gravity. They will also contribute to future space agriculture and terraforming technology, which uses plants to transform harsh planetary environments into ones suitable for humans.
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